A breeding colony of Macaca mulatta is maintained, providing pregnancies of known gestational age, term infants, and immature animals to NICHD-funded investigators, and breeding stock for future research programs. The colony totals 234 animals, 52 animals in corn cribs and 5 infants and juveniles being weaned in the indoor colony. During 1995 the conception rate was 77.10%. The prenatal mortality rate was 15.84%, infant and adult mortality rates were 7.81% and 5.40%, respectively. During 1995, 27 animals were loaned to investigators. 66.66% of the adult animals are housed in the indoor time-mated breeding program. Surplus juveniles are reared in group cages outdoors until sexual maturitywhen they are recycled into the indoor time-mated program. Additional replacements are produced and reared in the random-mated colony housed in a half acre field corral. There are currently 28 infants and juveniles in the corral and 19 infants and juveniles in the corn cribs. These will be available for breeding in several years. Attempts to enhance postnatal development via prenatal (intrauterine) treatment with epidermal growth factor (EGF) continue; significant effects on premature infant lung and gut development have been shown. In addition, transplantation of hematopoietic stem cells prenatally has established stable nonhuman primate chimeras in the absence of graft-versus-host disease. Further applications of ultrasound have successfully eliminated the need for extensive surgical procedures. Studies on the mechanisms of action of EGF on lung in prematurely delivered rhesus suggest that it may be useful in the treatment of human babies with respiratory distress syndrome. Data also suggest that a combined EGF/surfactant treatment regimen is superior to treatment with surfactant alone. Expansion of the reproductive monitoring program for breeding females has incorporated both ultrasonographic and endocrine techniques for the recognition, characterization, and determination of pregnancy and early pregnancy loss.